With the discovery of gravitational waves from coalescing black holes
[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}
(see also, e.g., [13]}, [14]}, [15]}),
binary neutron star [16]}, [17]}
and black hole – neutron star
[11]}, [19]}, [12]}
systems, we have entered a new era of compact object science. Especially
the matter present in the latter two systems, has the potential to fuel
electromagnetic counterparts, which where observed for the GW170817 event(
e.g.
[21]}, [22]}, [23]}, [24]}, [25]}, [26]}, [27]}, [28]}).
This makes the search for electromagnetic counterparts
an integral part of any gravitational wave detection. While the first
detection of a neutron star binary coalescence was accompanied by two
distinct counterparts [21]}, likely originating
during and after merger, there could be yet another counterpart emitted
right before merger [30]}, [31]}, [32]}.
Such a precursor is thought to arise due to interactions of
magnetic flux tubes in the pair-plasma filled common magnetosphere of the binary
[31]},which requires the presence a steady magnetic
field anchored in matter. Hence, making binary neutron star and black hole
– neutron star collisions prime targets to detect such precursors
[34]}. Indeed, in the case of black hole – neutron
star mergers, electromagnetic precursors might also be the only
accompanying transient if the neutron star is swallowed whole, see e.g.
[35]}, [36]}, [37]}, [38]}, [39]}.
As such, precursors are also potential candidates
for powerful radio signals
(e.g., [37]}, [41]}). The prospects for creating
early warning systems for electromagnetic follow-up observations of
precursor emission has recently been investigated
[42]}, [43]}, [44]}, crucial for their
potential future detection [45]}. For the event GW170817
attempts have even been made to detect precursor emission
[34]}, [47]}, see also
[48]}, [49]}. Coincident detection of precursor
emission could also be used to (further) constrain the sky localization of
the merger site[50]}.
